docs: add mention about importing modules [GispoCoding#35]

README.md

@@ -74,6 +74,8 @@ markers can be used.
   configure [`QgsApplication`](https://qgis.org/pyqgis/master/core/QgsApplication.html). With QGIS >= 3.18 it is also
   used to patch `qgis.utils.iface` with `qgis_iface` automatically.
 
+  > Be careful not to import modules importing `qgis.utils.iface` in the root of conftest, because the `pytest_configure` hook has not yet patched `iface` in that point. See [this issue](https://github.com/GispoCoding/pytest-qgis/issues/35) for details.
+
 ### Command line options
 
 * `--qgis_disable_gui` can be used to disable graphical user interface in tests. This speeds up the tests that use Qt